The invention relates to a sliding material of a copper alloy in which sliding material the resistance to fatigue and the anti-seizure property thereof are enhanced, a method of producing the same, and a sliding bearing made by use of the same.
Hitherto, there is known a Kelmet bearing which is used as a copper alloy sliding material. The Kelmet bearing comprises a back metal made of a steel, a bearing alloy layer bonded to the back metal which bearing alloy layer is made of a sintered alloy containing copper and lead, and an overlay layer bonded to the Cuxe2x80x94Pb-based bearing alloy layer of the sintered alloy, which Kelmet bearing is used as a sliding bearing for an automobile engine etc. In the Kelmet bearing, even if the overlay layer is worn away, Pb contained in the sintered alloy containing copper and lead exists on a sliding face, so that it keeps good anti-seizure property.
In the conventional copper alloy sliding materials represented by the Kelmet bearing, the anti-seizure property thereof is enhanced by adding Pb of about 20 mass percents. However, since Pb acts to cause bad influence on the environment, it is preferred to reduce the content thereof as little as possible or to use no Pb. However, since Pb has such function as explained above, the reducing of the content of Pb make the anti-seizure property deteriorated.
Further, in recent automobile engines, there is such a tendency as the temperature and specific load of the bearing become higher due to high rotation and high output design. However, since the Kelmet bearing contains such a high amount of Pb as to be about 20 wt. % which Pb has a low melting point and which Pb is soft, the Kelmet bearing has the problems that it is low in strength and that it is inferior in the resistance to fatigue particularly under high specific load.
The invention is achieved under this situation, and the object of the invention is to obtain the copper alloy sliding material which can bring about high anti-seizure property without using any Pb while having both of good anti-seizure property and good resistance to fatigue even under a high temperature and a high specific load, the method of producing the same, and the sliding bearing made by use of the sliding material.
The inventors of the invention have found out that, by providing such structure as both of soft copper alloy phase and hard copper alloy phase are in a mixture state in the sliding material of copper alloy, the anti-seizure property and the resistance to fatigue of the copper alloy sliding material are enhanced. Namely, on the surface of the sliding material in which the soft copper alloy phase and the hard copper alloy phase coexist in a mixture state, the hard copper alloy phase and the soft copper alloy phase become convex and concave in shape, respectively, when load is applied to the surface of the sliding material. In this state, since lubricant is retained in the concave portions, the anti-seizure property of the sliding material is enhanced. The inventors of the invention have further found out that, by making the hard and soft phases satisfy the condition of (H2/H1)xe2x89xa71.2 in which H1 is the Vickers hardness number of the soft copper alloy phase and in which H2 is the Vickers hardness number of the hard copper alloy phase, there occurs on these soft and hard copper alloy phases a concave and convex relation which can act as a lubricant reservoir.
Further, by making both of the soft and hard copper alloy phases coexist in the mixture state on the surface of the sliding material, the soft copper alloy phase acts to serve for the conformability of the sliding material, and the hard copper alloy phase acts to serve for the load applied to the sliding material. Usually, load borne on the hard phase acts as shearing stress at the boundary defined between the hard phase and the soft phase. However, in a case where both of the hard phase and the soft phase are formed of the same copper alloy, there occurs no clear boundary between the hard phase and the soft phase, so that the load borne on the hard phase is spread in area including the boundary between the hard and soft phases with the result that the resistance to fatigue of the sliding material is enhanced.
In view of this, according to a first aspect of the invention, the sliding material of the copper alloy has a structure in which both of the soft copper alloy phase and the hard copper alloy phase coexist in a mixture state, the Vickers hardness number of the hard copper alloy phase being not less than 1.2 times that of the soft copper alloy phase, whereby it becomes possible to obtain good conformability, superior resistance to fatigue, and superior anti-seizure property without adding any Pb.
According to a second aspect of the invention, since the copper alloy sliding material is made of a sintered alloy, the structure in which both of the soft copper alloy phase and the hard copper alloy phase coexist in a mixture state can be readily formed.
In the second aspect of the invention, the composition of the sintered alloy essentially consists, by mass, of: not more than 15% Sn; at least one kind of 0.2 to 40% in total selected from the group consisting of Ni, Co, Mn, and Fe; at least one kind of not more than 40% in total selected from the group consisting of P, Zn, Al, Ag, and Si, the total amount of the at least one kind selected from the group consisting of Ni, Co, Mn, and Fe and the at least one kind selected from the group consisting of P, Zn, Al, Ag, and Si being in the range of 0.2 to 40%; and the balance substantially Cu. The reasons for the numerical limitations disclosed above are explained below.
(a) Sn not more than 15 mass %:
Sn acts to strengthen the matrix and to enhance the resistance to fatigue. In addition, the adding of Sn enhances the anti-seizure property. In a case where the amount of Sn exceeds 15 mass %, much amount of Cuxe2x80x94Sn intermetallic compound occurs to make the sliding material brittle.
(b) At least one kind of element of 0.2 to 40 mass % in total selected from the group consisting of Ni, Co, Mn, and Fe:
The at least one kind of element strengthens the matrix and enhances the resistance to fatigue. Further, since it is an element which hardly migrates insofar as the sintering temperature of 800 to 920 is concerned, the at least one kind of element exists essentially in the hard copper alloy phase and acts favorably for providing hardness difference between the hard and soft copper alloy phases. In a case where the total amount of the at least one kind of element is less than 0.2 mass %, it is impossible to make the hard copper alloy phase hard in hardness. In another case where it exceeds 40 mass %, the sliding material becomes apt to adhere to a counterpart shaft, so that it causes bad influence on the anti-seizure property.
(c) At least one kind of element not more than 40 mass % in total selected from the group consisting of P, Zn, Al, Ag, and Si:
The at least one kind of element strengthens the matrix and enhances the resistance to fatigue. Further, since it is an element which readily migrate insofar as the sintering temperature of 800 to 920 is concerned, the at least one kind of element acts to strengthen the whole sliding material made of the copper alloy. In a case where the total amount thereof exceeds 40 mass %, the whole sliding material made of the copper alloy becomes excessively hard in hardness unfavorably.
(d) The total amount of both of the at least one kind of element selected from the group consisting of Ni, Co, Mn, and Fe and the at least one kind of element selected from the group consisting of P, Zn, Al, Ag, and Si, being in the range of 0.2 to 40 mass %:
In a case where the total amount of the elements selected from the two groups exceeds 40 mass %, the whole of the sliding material of the copper alloy becomes excessively hard in hardness, which is unfavorable when the sliding material is used as a bearing material.
According to a third aspect of the invention, since the at least one kind of element selected from the group consisting of Ni, Co, Mn, and Fe is contained in the hard copper alloy phase, it becomes possible to readily provide the hardness difference between the hard and soft copper alloy phases.
Unless relatively large difference in hardness is provided between the hard and soft copper alloy phases, it becomes impossible to provide the condition that the Vickers hardness number of the hard copper alloy phase is not less than 1.2 times as much as that of the soft copper alloy phase. To meet this condition, it is preferred that the at least one kind of element (, strengthening element,) for strengthening the hard copper alloy phase does not diffuse into the soft copper alloy phase. However, in a case where none of the strengthening elements is diffused into the soft copper alloy phase, there occurs excessively clear boundary between the hard and soft copper alloy phases, which is not favorable in view of the resistance to fatigue.
Regarding this respect, in a fourth aspect of the invention, the two or more kinds of copper alloy powders are mixed, or pure copper powder and at least one kind of copper alloy powder are mixed, and then the mixture is sintered, wherein the at least one kind of copper alloy powder is of such a copper alloy as to contain the at least one kind of element (Ni, Co, Mn, and/or Fe) for increasing the hardness, so that it becomes possible to readily form both of the hard copper alloy phase and the soft copper alloy phase and to readily provide a predetermined difference in hardness therebetween. In addition, in this case, since both of the hard phase and the soft phase are of the copper alloy, both of the soft phase and the hard phase are good in wettability, so that the strengthening element contained in the hard phase diffuses into the soft phase with the result that the boundary defined therebetween does not become excessively clear regarding difference in hardness.
According to a fifth aspect of the invention, a heat treatment is performed for 1 to 20 hours at a temperature of 200 to 500xc2x0 C. after the final step of the sintering, so that the hardness of one kind of copper alloy phase such as, for example, Cuxe2x80x94Snxe2x80x94Ni alloy phase increases because of the age hardening thereof while in another kind of copper alloy phase such as, for example, Cuxe2x80x94Sn alloy phase no age hardening occurs, whereby the difference in hardness can be readily provided between the soft copper alloy phase and the hard copper alloy phase. Further, even in a case where no age hardening occurs in the hard copper alloy phase, since the temperature of softening occurring during the heat treatment performed after the work hardening differs one copper alloy phase by one copper alloy phase, it becomes possible to make only the soft copper alloy phase soft in hardness through the heat treatment performed after the final rolling of the copper alloy by suitably selecting the compositions of the hard copper alloy phase and the soft copper alloy phase, and it is also possible to make the difference in hardness larger.
According to a sixth aspect of the invention, since the copper alloy sliding material is bonded onto the back metal to thereby form a bearing, it can be used as a sliding bearing for an automobile engine of high rotation and high output.
According to a seventh aspect of the invention, since the overlay layer made of metal and/or resin is bonded onto the surface of the sliding material of copper alloy, it becomes possible to provide a sliding bearing superior in each of initial conformability, foreign matter embeddability, and anti-seizure property.